Economic conditions demand the recycling of used materials. In the production of paper it has been known that used paper may be deinked, bleached, and made into a slurry suitable for the production of new paper. The proportion by weight of printing inks in the paper to be recycled is relatively small and the ink must be removed to produce paper of the desired brightness.
The technology involved in deinking according to the froth flotation process has been known for some time. Presorted waste paper is defibred by means of a pulper in preheated water with the addition of suitable deinking chemicals such as alkalis, bleaching agents, collectors, and foamers. By the action of the various chemicals the adhesives in the printing inks are removed so that the inks can be separated from the fibers.
The printing inks are removed in the so-called froth flotation cells operating under the principle of froth floation. The treatment of the waste paper slurry with chemicals prepares the materials for the froth flotation process. The slurry is aerated and the air bubbles, which have been stabilized by the addition of flotation agents, rise through the slurry and pigments accumulate on the surface of the bubbles due to the differing wetability and are carried to the surface of the liquid. The ink particles accumulate in the foam or froth formed over the surface of the liquid and can be skimmed off from the fiber suspension.